The present invention relates to a method of and an apparatus for measuring and evaluating the strength of a film material by measuring the peel strength of a fragile thin film. In more detail, this invention relates to a method of and an apparatus for measuring and evaluating the strength of a material, thereby measuring the peel strength as well as the fragility breaking strength of a wear resistant thin film, such as an electrically insulating thin film which might be a diamond-like carbon film useful as a protective film for protecting a magnetic recording layer of a hard disk.
Since a vertical force and a tangential friction force are usually applying between the head of a hard disk and protection film on a magnetic recording layer, it is necessary to improve the fragility breaking strength against the vertical stress of the film, as well as to improve the peel strength of the film caused by the tangential force. To improve them, more precise measurement technique is required to be developed. Here, the peel strength means an interface strength between a thin film and the substrate, while the fragility breaking strength means a crack starting strength at the front end of an indenter when the indenter is vertically pressed into the film.
When measuring the peel strength of the above-described fragile film, an indenter is pressed into the surface of the film, moving downward from an upper position. Alternatively, a substrate on which a piece of film has been formed is set in an inclined position, while the indenter is pressed into an interface between the film and the substrate. The point at which a sudden and rapid change in an indentation load of the indenter happens is considered that the fragile film begins to peel from the substrate. When measuring the peel strength, a relationship between an indentation load of the indenter and an indentation displacement is obtained, thereby calculating a film peel strength in accordance with a load detected when there occurs a sudden and rapid change in the indentation load.
During a material test for evaluating a fragility breaking strength of a fragile material, an indenter is pressed into a piece of test material, thereby measuring an AE (acoustic emission) which occurs when a fragility breaking occurs, thus determining the occurrence of a breakage (for example, refer to Japanese Unexamined Patent Application Publication No. 6-341942).
However, in the above-described method by measuring an indentation displacement, there has been a problem that thee curve of a relationship between an indentation load and an indentation displacement gives is a large and unclear change, thus bringing about a considerable error in each measurement result. Moreover, with regard to the material test by measuring AE, it is required to employ a large scale apparatus for distinguishing between an AE and a noise sound.
It is an object of the present invention to provide an improved method and an improved apparatus for measuring and evaluating the strength of a material, the method and the apparatus are so formed that they can detect charged particles generated when the aforementioned film peels and a material fragility breakage occurs, thereby high-sensitively specifying a peel occurring time and a fragility breaking time, thus making it possible to correctly measure the peel strength and the fragility breaking strength.
In order to achieve the above object, the present invention provides a material strength measuring and evaluating method for measuring and evaluating a peel strength and/or a fragility breaking strength of a fragile thin film. The method comprises the steps of pressing an indenter into a test object and measuring an indentation load as well as an indentation depth, while at the same time detecting charged particles emitted from a peel starting point or a breakage starting point; specifying a peel occurring time and a fragility breaking time when charged particles are increased; measuring a peel strength and/or a fragility breaking strength.
In the above-described method, the test object may be a fragile film itself and fragile thin film covering the substrate. Further, the sample setting surface on the sample mounting base is changeable between a horizontal position and an inclined position, and the test object may be positioned horizontally while the indenter may be vertically pressed into the surface of the test object. Moreover, the test object may be arranged to form a tilt angle with the pressing direction of the indenter, so that the indenter may be pressed in a direction inclined with respect to the surface of the test object. In addition, when charged particles are collected by the charged particle collecting element, an electric potential having a polarity opposite to that of the charged particles (to be collected) is applied to the charged particle collecting element.
Furthermore, the present invention also provides a material strength measuring and evaluating apparatus for use in carrying out the aforementioned method. Such an apparatus comprises a sample mounting base for mounting a test object; an indenter to be pressed into the test object; a charged particle collecting element disposed in the vicinity of the front end portion of the indenter and formed integrally with or independently front the indenter, an indentation load detector for detecting an indentation load of the indenter; a displacement detector for detecting a displacement amount of the indenter; signal processing systems for measuring a peel strength at the time of peel occurrence and/or a fragility breaking strength at the time of fragility breaking, in accordance with the output signals fed from the indentation load detector, the displacement detector and the charged particle collecting element.
In particular, it is preferable that the front end portion of the indenter is formed by a diamond, a sapphire or a piezo-electric material. Further, it is preferable that the indentation load detector is an electronic balance positioned below the test object, while the displacement detector is a light reflection intensity meter or a light interference meter, each capable of measuring a relative displacement of the indenter with respect to the test object.